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***CC_Guy*** · (11-10-2014)

It looks like the adhesion issue was between the adhesive and the panels. There are few places shown in the picture where there's no adhesive on the chassis.

***travelite*** · (This post was last modified: 11-10-2014 21:59 by travelite.)

My thoughts exactly CC_Guy. It's virtually a clean break on the panel side. This would lead me to believe that an incompatible glue was selected, or the surface preparation was defective, or the process setup times weren't followed, as opposed to an inadequate shear or tension strength of the adhesive itself. Sounds like this issue could be remedied.

I don't think thermal expansion of the stainless panels is much of an issue. A quick check shows expansion at 0.0000096 in/in deg F. Assuming the panels are around 100" long. That would be about 0.01" of expansion for every 10 deg F increase in temp. A 100 deg F temperature swing only results in 0.1" expansion. You can reason along these same lines and conclude that bus chassis flex isn't the issue either.

I calculate that each panel should weigh less than 100lbs which puts very little shear stress on the adhesive. Here's some specs on Loctite 5590 which give a shear yield stress of 290 psi.

Hi Arctic Bird, It doesn't take long for rust to form when salt road spray is flowing behind your panels. I looked at a coach that was parked at the beach and the internal basement beams were a mess. There was a panel leak that allowed corrosive water to flow into the basement.

Hisham Amaral · (This post was last modified: 11-11-2014 12:19 by Hisham Amaral.)

(11-08-2014 12:11)CC_Guy Wrote: The re-glue process was Prevost's first fix. According to some, that approach failed, at least it did on some number of repairs. Then came the rivet approach that also required a change in adhesive. As I understand it, this approach too has failed. I don't know what the number of failures where after either fix and whether they were the result of workmanship in the repairs or actual failures of the adhesive or were do to some other issue. These could be isolated cases that can be explained. I just don't know, so I only throw this out as information I've heard.

With the advancements in adhesives and the fact that similar bonding has been done in other industries, it's hard to understand why this failure has occurred and why it has affected several model years. To suggest why the failures have occurred would simply be speculation on my part. What I find disturbing is that I have never heard of a single XL-II owner ever being notified by Prevost or their converter that they should have their coach tested for delamination. Not notifying owners, if that is in fact true, has to be a move now regretted. Unexpected failures or other defects happen. How you handle them is critical. Touting a million mile chassis and not notifying owners with far less than a million miles has to have a negative effect on new sales and certainly resale.

BTW, Prevost initially tested glued on panels with the passenger door on late model XLs. I've never heard of a single one of them failing (knock on wood). So, adhesives can work.

(11-10-2014 21:03)travelite Wrote: My thoughts exactly CC_Guy. It's virtually a clean break on the panel side. This would lead me to believe that an incompatible glue was selected, or the surface preparation was defective, or the process setup times weren't followed, as opposed to an inadequate shear or tension strength of the adhesive itself. Sounds like this issue could be remedied.

I don't think thermal expansion of the stainless panels is much of an issue. A quick check shows expansion at 0.0000096 in/in deg F. Assuming the panels are around 100" long. That would be about 0.01" of expansion for every 10 deg F increase in temp. A 100 deg F temperature swing only results in 0.1" expansion. You can reason along these same lines and conclude that bus chassis flex isn't the issue either.

I calculate that each panel should weigh less than 100lbs which puts very little shear stress on the adhesive. Here's some specs on Loctite 5590 which give a shear yield stress of 290 psi.

Hi Arctic Bird, It doesn't take long for rust to form when salt road spray is flowing behind your panels. I looked at a coach that was parked at the beach and the internal basement beams were a mess. There was a panel leak that allowed corrosive water to flow into the basement.

David
In my opinion the issue is both manufacturing and engineering. With failure numbers that high process cannot be the only botched up job Engineering is also a large contributor. BTW each panel is 96in by 32in by I think .06IN with mass density of .283 LB/CUIN that is about 52 lbs for each panel, if the the thickness of the material is .060 in.

***travelite*** · 11-12-2014, 00:17

(11-11-2014 12:13)Hisham Amaral Wrote: David
In my opinion the issue is both manufacturing and engineering. With failure numbers that high process cannot be the only botched up job Engineering is also a large contributor. BTW each panel is 96in by 32in by I think .06IN with mass density of .283 LB/CUIN that is about 52 lbs for each panel, if the the thickness of the material is .060 in.

Thanks Hish,

I really appreciate your expertise in this area. Knowing what we know about the numbers of the coaches effected, with your guidance it's clear that there's a combination of process and engineering issues going on here - manufacturing can only work with the materials selected by engineering.

Jerry_94110 · 11-13-2014, 22:33

When shopping for an XLII how do you determine if the panels are loose, have been loose and already repaired or manufactured correctly from the start? Is there any particular area to check?

Thanks,
Jerry Kocher
1987 Bluebird PT 38
San Francisco, CA

Hisham Amaral · (11-14-2014)

(11-13-2014 22:33)Jerry_94110 Wrote: When shopping for an XLII how do you determine if the panels are loose, have been loose and already repaired or manufactured correctly from the start? Is there any particular area to check?

Thanks,
Jerry Kocher
1987 Bluebird PT 38
San Francisco, CA

Jerry
Tap on each panel just like you would trying to install a picture frame in a wall. if you get a hollow sound that tills you there no connection to base formers. the adhesive have separated from the SS and the base.
Jerry did make it to Pine Mountain ralley last month?

***CC_Guy*** · 11-14-2014, 17:19

Jerry,

You might be able to call Prevost with the VIN and they may tell you if it was repaired with the adhesive only repair. If it was fixed with the second repair process, you can tell by looking. Instead of side panels that but up to one another you will find a trim strip over the panel seem. Although you can't see them, rivets are under the trim cover.

The million dollar question would be, how can you determine if the adhesive will fail later? The answer is either there's no way of telling or they will all eventually fail. I have no idea which answer is correct.

Jerry_94110 · 11-14-2014, 19:04

Thanks guys this help......

(11-14-2014 11:56)Hisham Amaral Wrote: Jerry did make it to Pine Mountain ralley last month?

@ Hisham, no I did not attend the Pine Mountain rally. A bit too far for me. Well, if I didn't have to work, I wouldn't mind the distance.

***travelite*** · (This post was last modified: 11-15-2014 14:15 by travelite.)

(11-14-2014 17:19)CC_Guy Wrote: The million dollar question would be, how can you determine if the adhesive will fail later? The answer is either there's no way of telling or they will all eventually fail. I have no idea which answer is correct.

Hi CC_Guy,

We'd like the bonded joint to last as long as the bulk of the adhesive (cohesive failure) as opposed to early failure due to substrate separation (adhesive failure) - adhesive failure seems to be the case in the photo I posted earlier.

Prevost has been using a single part polyurethane, either Sikaflex 221 or 255. Here's a list of the products used on my bus panel when I had a repair done to the converter modified panel under my bedroom slide:

Sika Cleaner-205
Sika Activator-205
Sika Primer-206 G+P
Sikaflex 255 FC
Bostik 70-03A

Arctic Bird posted about the heavy use of adhesive in the earlier pic. Single part polyurethane's require water and oxygen to cure. When the thickness of the bead goes beyond 1/2" the curing stops due to a lack of moisture. Manufacturers typically recommend small diameter beads of adhesive. This allows curing - also thinner beads elongate better than thicker beads. This may be why Prevost has moved to a two part Loctite epoxy, part number 5590. Two part epoxies continue to cure even if they've been covered or painted over and sealed from the atmosphere.

Something I noticed when Prevost was doing my panel is that they paid a lot of attention to panel and bus framework prep. Both sides were cleaned, activated, and primed with the recommended Sika products. Gloves were worn and panels were cleaned and untouched until mating time. Once the adhesive bead was applied the panel was put in place in a timely manner.

The skin on our buses reminds me of the "curtain walls" used in modern day skyscrapers. Exterior building panels are glued in place. They're not structural but they do need to withstand movement, expansion, vibration, UV, and temperature swings, and they do need to be weather tight. I'm not a building structure expert but I understand that at least one building code allows a degree of water infiltration as long as an exit path exists. I've always felt the same way about our skin. The adhesive is there primarily to hold the panels in place, and by that definition there is no failure. It's also true that the adhesive is serving double duty in that it's being asked to also act as a weather barrier. I'd like to see a design that separates the two functions with an easily replaced weather seal that can be serviced every so many years.

Even with proper material selection and perfect preparation and installation, bonded joints have a finite life span and will forever require regular inspection, repair, and replacement. It's not totally unlike rivets where rivet holes are areas of weakness where stress cracks can form and which have to be periodically checked for corrosion or loosening. It'd be great if Prevost could make these procedures as easy and painless as possible. A possible step in that direction with adhesives would be to separate the mechanical fastening system from the weather sealing system.

Back to your million dollar question. I think we have a pretty good idea of a typical cohesive failure interval, what we don't know is what the chances are of an early adhesive failure in a randomly selected XLII. I seem to recall at one point, circa 2013, Prevost Jacksonville saying there were something like 250 buses with early failure. If it hasn't suffered adhesive failure yet, then IMO it'll probably make it to cohesive failure.

***CC_Guy*** · 11-15-2014, 11:33

I have never come across or heard mention of the Bostik product you referenced. I looked it up and it would seem like a great product for sealing roof penetrations and seems when you don't want or need a self leveling product. Even its price is competitive with commonly used roof sealants.

Has anyone else come across the use or recommendation of this Bostik product? I've been using Sikaflex 221 and it's a PITA to tool and really isn't the best when it comes to UV resistance. Sikaflex does have a similar UV resistant product, but it's harder to come by.

Just curious